The Fundamental Mechanics of XSS

To truly understand XSS, we need to grasp how web browsers interpret and execute content. Browsers are designed to be helpful, automatically executing any JavaScript code they encounter while rendering web pages. This feature, while essential for modern interactive websites, becomes a severe vulnerability when untrusted content is mixed with legitimate code. When an attacker successfully injects malicious JavaScript into a web page, the browser has no way to distinguish between legitimate scripts from the website and injected malicious scripts. Both execute with the same privileges, accessing cookies, session tokens, and other sensitive information.

Consider a simple example: a comment section on a blog. When users submit comments, the website stores them in a database and displays them to other visitors. If the website doesn't properly sanitize these comments, an attacker could submit a comment containing JavaScript code like <script>alert('XSS')</script>. Without proper protection, when other users view this comment, their browsers will execute this script. While an alert box is harmless, the same technique could be used to steal cookies, redirect users to malicious sites, or perform actions on behalf of the victim.